Electrolyte and process for electroplating black chromium and article thereby produced



United States Patent 3,419,481 ELECTROLYTE AND PROCESS FOR ELECTRO-PLATING BLACK CHROMIUM AND ARTICLE THEREBY PRODUCED John B. Nelson, Jr.,Chardon, Ohio, assignor to Diamond Shamrock Corporation, a corporationof Delaware No Drawing. Filed Apr. 8, 1966, Ser. No. 541,075 7 Claims.(Cl. 204-51) This invention relates to a composition and method for theelectrodeposition of chromium-containing coatings on a conductive metalsubstrate as the cathode and more particularly relates to theelectrodeposition of chromium coatings on such substrates in suchphysical and chemical form as to produce a black surface having a highdegree of absorptivity for both heat and visible light.

Black chromium deposits find use in areas where their heat and lightabsorbing properties are important, one example of which is the militaryfield where such items as firearms, communications equipment, personnelornament, etc. are so coated. Such deposits are also valuable fordecorative purposes such as on metal furniture, automobile parts,plumbing fixtures, etc., where their corrosion resistance coupled withtheir appearance make them superior to other black finishes such aspaint.

While several processes and plating bath compositions have heretoforebeen disclosed in the art for producing black chromium electrodeposits,they have not received wide commercial acceptance for a variety ofreasons. For example in one process, a grey-black deposit is obtainedfrom a chromic acid bath having a low (less than 0.07%) sulfate contentto which has been added a small amount of a carboxylic acid, preferablyacetic acid. The chief disadvantages of this bath arise from theconditions for electrodepositing the black chromium whereby very highcurrent densities, i.e., 10,000 to 20,000 amperes per square meter (935to 1870 amperes per square foot), must be used at relatively lowtemperatures, i.e., about 20 C. Therefore, in addition to the excessiveconsumption of current, refrigerating means must be provided to maintainthe proper temperature in this process.

It has also been disclosed that dark grey to black electrodeposits maybe obtained from a chromic acid bath from which the sulfate has beenremoved and which contains large amounts of acetic acid. While this bathis said to operate at low current densities, its effective currentdensity range is quite limited, i.e., 40 to 90 amperes per square foot,and this bath also has the further disadvantages that steam coils mustbe provided to maintain the desired temperatures for plating 90 to 115F., and that an exhaust fan is required to remove the noxious aceticacid vapors.

In a more recent process, a black chromium deposit is obtained from anaqueous bath consisting of chromic acid and a fluoride catalyst. Whilethe current density and temperature ranges recommended are satisfactory,reproduction of good results is still quite ditficult to attain becauseof the criticality of the fluoride catalyst concentration and thenecessity according to this teaching of eliminating all other catalyticions from the bath. Thus, in addition to precluding the presence ofsulfate in the bath, it is also necessary to use deionized or distilledwater for bath make-up and pre-bath rinsing procedures in order that noforeign ions be introduced. Consequently this type of bath is ditficultto control and not suitable for large scale commercial applications.

It is an object of the present invention to provide an improved bathcomposition and easily operable process whereby uniform, black,chromium-containing electrodeposits may be obtained.

It is another object of the present invention to provide an improvedbath composition which is highly effective over a wide range ofoperating conditions for electrodepositing black, chromium-containingcoatings.

These and other objects of this invention will become apparent from thespecification and claims that follow.

It has now been found that uniform, corrosion resistant, black,chromium-containing coatings may be electro-deposited on an electricallyconductive member by making said member the cathode in an aqueoussolution consisting essentially of from at least 60 grams per liter upto saturation of chromic acid, a fluoride-containing catalyst in anamount sufficient to supply from about 0.03 to about 1 gram fluoride insolution per liter and an inorganic nitrogen-containing compound in anamount sufficient to supply the equivalent of from about 0.35 to about3.5 grams of (N0 radical per liter, said aqueous solution being free ofsulfate ions, and passing a direct current between said cathode and ananode immersed in said solution at a current density of from about 30 to1500 amperes per square foot while maintaining the solution at atemperature of from about 60 to F.

While solutions containing chromic acid and a fluoridecontainingcatalyst have previously been used to obtain black, chromium-containingelectrodeposits, it has now been found that the inclusion of relativelysmall amounts, i.e., amounts suificient to provide the equivalent offrom about 0.35 to 3.5 grams of (N0 radical per liter of an inorganicnitrogen-containing compound as defined below, affords substantialadvantages. With the use of such inorganic nitrogen-containing compoundsa uniform, black, chromium-containing electrodeposit is readily obtainedover a wide range of current densities. The deposit itself is alsoimproved in appearance in that it is darker in color and has a glossierfinish than that obtained without the use of the inorganicnitrogen-containing compounds.

Any commercially available grade of chromic anhydride (CrO may be usedin the practice of this inven tion, but since commercial chromicanhydride generally contains significant quantities of sulfates whichinterfere with the formation of the desired black, chromium-containingdeposits, it is necessary that the chromic acid solutions describedherein be treated before use to remove these sulfates. This treatmentmay be easily eifected by the addition to the solution of a source ofbarium ion such as barium carbonate or barium oxide. The addition of 5to 20 grams per liter of barium carbonate will generally provide asufficient excess to insure a sulfate-free plating solution. The amountof chromic anhydride used may be within the range of from at least 60grams per liter up to saturation, preferably about 300 to 500 grams perliter. Particularly preferred at this time is a concentration of 450grams per liter.

Any of the fluoride-containing chromium plating catalysts known to theart which supply fluoride in solutions of chromic acid may be used inthebath compositions of this invention. Examples of these catalysts arehydrofluoric acid, fluoboric acid, fluosilicic acid and water solublealkali metal, alkaline earth metal, heavy metal and ammonium saltsthereof. Preferred fluoride-containing catalyst are those obtained byreacting a hexavalent chromium compound such as chromic acid, an organicreducing agent such as sucrose and a fluoride-silicon compound such asfiuosilicic acid as described in US. Patent No. 2,841,540. The amountsof fluoride-containing catalyst useful in this invention will varyaccording to the amount of fluoride which the catalyst can supply to thechromic acid solution, and the amount of .fluoride in solu-- tion may bevaried from about 0.03 to about 1 gram per liter, preferably from about0.1 to 0.25 gram per liter.

The inorganic nitrogen-containing compounds effective in the solutionsof the present invention are nitric acid and nitrous acid and the alkalimetal, alkaline earth metal and ammonium salts thereof, nitric acid andsodium nitrate being preferred at present by reason of their readyavailability and comparatively low price. These inorganicnitrogen-containing compounds are effective for the purposes of thepresent invention in amounts sufficient to supply the equivalent ofabout 0.35 to about 3.5 grams of (N radical per liter, preferably about0.7 to 2 grams of (N0 radical per liter. While amounts in excess of 3.5grams per liter (N0 may be used, they are without apparent additionalbeneficial effect. Thus, by way of example, if sodium nitrite is used inthe practice of this invention amounts within the range of from about0.52 to about 5.2 grams per liter, preferably 1 to 3 grams per literwill be effective; if sodium nitrate is used amounts within the range offrom about 0.65 to about 6.5 grams per liter, preferably 1.3 to 3.7grams per liter will be effective.

The plating solution disclosed herein for electro-depositing black,chromium-containing coatings are easier to control than prior artsolutions in that it is not necessary to use deionized or distilledwater in either the makeup of the solutions or in the rinsing stepsprior to immersion of the article to be coated in the solution. Thesolutions may be used effectively in producing electrodeposits for longperiods of time without detrimental effects due to decompositionproducts and employing only the precautions used in the operation ofconventional chromium plating solutions to prevent the introduction offoreign ions.

Uniform, black, chromium-containing deposits are readily obtained over acurrent density range of about 35 to 1500 amperes per square foot. Thetemperature of the bath may be maintained within the range from about 60to 130 F., preferably from 70 to 95 F., thereby eliminating the need foreither heating or cooling means in most cases.

While the time required to electrodeposit the black, chromium-containingcoatings will vary somewhat depending on the solution composition,temperature, and the thickness of coating desired, 1 to 5 minutes at acurrent density within the above mentioned range will generally give auniform, black, chromium-containing deposit having good corrosionresistance.

Any of the insoluble anodes used with conventional chromium platingbaths may be employed. Especially to be preferred are lead or lead alloyanodes. Corrosion and erosion of these lead or lead alloy anodes whenused in the practice of this invention is on a level equivalent withthat experienced in conventional chromium plating solutions. Tanksemployed for containing the solutions of the present invention may belined with any suitable corrosion resistant material such as glass,ceramic material, polyvinyl chloride, lead and the like.

While the best results are obtained by subjecting the electricallyconductive member to be pated by the process of this invention to anickel strike treatment, i.e., cathodically plating nickel on saidmember for a short period at any conventional current density, quitesatisfactory coatings may be obtained by plating directly onto copper,zinc, iron, steel, stainless steel, conventional chromium deposits andother conductive bases.

As is the practice in conventional chromium plating, mists suppressantsmay be added to the solution before plating. Many of the commerciallyavailable products for this purpose (which are generally proprietarysurfactantcontaining compositions) are acceptable.

Compositions of this invention are ready for the electrodepositingoperation immediately upon dissolution of the proper amounts of chromicanhydride, fluoride-containing catalyst and inorganic nitrogencontaining compound in tap water, order of addition not being important,without pretreatment or preliminary electrolysis of any kind other thanthe addition of excess barium to remove the sulfate.

In order that the compositions and process of this invention may be morereadily understood by those skilled in the art, the following specificexampes are provided. Unless otherwise, noted, the examples that followare the result of evaluation of the compositions and process of thisinvention in a modified 267 milliliter Hull cell using as the cathodebrass panels, 2.5 by 4 inches, which have been given a thin, uniformnickel coating by electrodebeen given a thin, uniform nickel coating byelectrodepsition from a commercial bright nickel plating bath. Thestandard 267 milliliter Hull cell is a trapezoidal box of non-conductivematerial in the opposite ends of which are positioned anode and cathodeplates as is more particularly described in US. Patent No. 2,149,344. Bythe use of this device it is possible to easily determine the effectiveplating range of a plating composition under varying conditions. Thecurrent density at any point on the cathode is determined according tothe formula A:C (27.7-48.7 log L) wherein A is the current density atthe selected point, C is the total current applied to the cell and L isthe distance of the selected point from the high current density end ofthe plate. In the modified version used herein, holes are provided inthe sides of the Hull cell adjacent the anode and cathode whereby, uponimmersion of the cell in a container of plating solution, improvedelectrolyte circulation and consequently improved temperature control isafforded as is more particularly described in an article by J.Branciaroli appearing on page 257, March 1959 issue of Plating, vol. 46,No. 3 (a publication of the America Electroplaters Society, Inc.).

EXAMPLE 1 An aqueous plating solution is made containing 450 grams perliter of CrO 1 gram per liter of fluoride-containing catalyst (thereaction product of a chromium compound, an organic reducing agent andfluosilicic acid as more particularly described in US. Patent No.2,841,540 and containing by weight chromium, 25 by weight fluoride and16% by weight silicon) and 7.5 grams of BaCO to precipitate the sulfate.A panel is electroplated for 3 minutes at 10 amperes and a bathtemperature of 96 F. A dull black electrodeposit is obtained from thehigh current density end of the panel to a current density of about 150amperes per square foot, a value corresponding to 46% coverage.

To the above bath is added 1.5 ml. of concentrated nitric acid (specificgravity 1.42 gm./ml.). A second panel is now electroplated for 3 minutesat 10 amperes and a temperature of 96 F. The black deposit range is nowfound to extend to about amperes per square foot or 78% coverage. Inaddition to the substantially improved plating range the deposit itselfis more uniform, is darker and has a glossier appearance than thedeposit obtained without the addition of the nitric acid.

EXAMPLES 2-9 A series of Hull cell panels is run as indicated inTable 1. In each instance the solution is treated with an excess of BaCOto precipitate the sulfate and the fluoride-containing catalyst is thesame as described in Example 1. The column titled Range indicates theeffective plating range obtained, i.e., it indicates that a uniformblack deposit is obtained from the extreme high current density end ofthe panel to the area on the panel corresponding to the stated value.

TABLE 1 Ex. CrO a F-Catalyst HNOa Temp. Range (gram/liter) (gram/liter)(ml./liter) F.) (a.s.l.)

1 Specific gravity 1.42 gin/ml. 2 Amperes per square foot.

These examples show the effect of increasing temperature on theeffective plating range of the compositions of this invention andindicate that while a loss of coverage in the low current density areasoccurs with increasing temperatures, this may be corrected by increasingthe amount of nitrogen-containing compound. At the high temperature ofExample 8, a grey band appeared in the deposit at the low currentdensity area. This band disappeared in Example 9 wherein more HNO' wasadded, with only a slight loss in coverage. Thus, the present inventionis shown to yield a uniform black electrodeposit over a broad currentdensity range even with widely varying plating solution temperatures.

EXAMPLE 10 A panel is electroplated in a plating solution containing 450grams per liter Cr 1 gram per liter of the fluoridecontaining catalystof Example 1, 1.5 ml. of concentrated HNO (specific gravity 1.42gm./ml.) and BaCO in excess of that required to precipitate the sulfatepresent. The bath is maintained at 112 F. for 3 minutes While applying30 amperes direct current. A uniform, black, chromium-containing depositis obtained over an effective plating range of from greater than 1500'amperes per square foot to about 30 amperes per square foot.

EXAMPLE 11 To illustrate the use in this invention of inorganicnitrogen-containing compounds other than HNO a plating solution isprepared as follows: 450 grams per liter CrO 1 gram per liter fluoridecatalyst (as in Example 1); 2.5 grams per liter Ca(NO -4H O and excessBaCO over that required to precipitate the sulfate. A panel is platedfrom this bath in a Hull cell at 78 F. for 3 minutes with an appliedcurrent of amperes. Although there is some evidence of burning at theextreme high current density end of the panel, a uniform black depositis obtained to a current density of about 70 amperes per square foot.

EXAMPLE 12 To show the eifectiveness of the compositions of thisinvention over a wide range of CrO concentrations the following solutionis used: 300 grams per liter CrO 1 gram per liter fluoride catalyst (asin Example 1), 2.9 grams per liter of Ca(NO -4H O and excess BaCO overthat required to precipitate the sulfate present. The panel is platedfor 3 minutes at 72 F. and 10 amperes applied current. A black depositis obtained to the area on the panel corresponding to about 120 amperesper square foot with only a slight burn at the extreme high currentdensity edge.

Now 300 grams per liter of CrO is added (total 600 grams per liter) andanother panel is plated at 81 F. for 3 minutes. An excellent blackdeposit is obtained to 60 amperes per square foot and the burned area isgreatlv decreased.

EXAMPLE 13 To show the effectiveness of inorganic nitrogen-containingcompounds other than nitric acid and salts thereof the following platingsolution is used: 450 grams per liter CrO 1 gram per liter fluoridecatalyst (as in Example 1) and excess BaCO to ensure complete sulfateprecipitation. A panel is plated for 3 minutes at 78 F. and 10 amperesapplied current. A black deposit is obtained only to about 200 amperesper square foot with considerable burning on the high current densityend of the panel. This corresponds to a coverage of about 40%.

Now 1 gram per liter of NaNO is added and a panel is plated under thesame conditions as above. A uniform black deposit is obtained to acurrent density of about 60 amperes per square foot with no burn on thehigh cur- EXAMPLE 14 To show the use of another fluoride-containingcatalyst in the practice of this invention an aqueous solution is usedas follows: 450 grams per liter CrO 2 ml. of concentrated HNO (specificgravity 1.42 grams per milliliter), 0.3 ml. of HF (49% aqueous solution)and an excess of BaCO over that required to precipitate the sulfatepresent. A Hull cell panel is plated for 3 minutes at 73 F. and anapplied current of 10 amperes. A black deposit is obtained from the highcurrent density end of the panel to about 70 amperes per square foot.

It is to be understood that although the invention has been describedwith specific reference to particular cmbodiments thereof, it is not tobe so limited since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

What is claimed is:

1. A composition of matter for use in the electro-deposition of black,chromium-containing deposits which is an aqueous solution consistingessentially of from about 60 grams per liter to saturation of chromicanhydride, a fluoride-containing catalyst in an amount sufiicient tosupply from about 0.03 to about 1 gram of fluoride in solution perliter, an inorganic nitrogen-containing compound in an amount suflicientto supply the equivalent of from about 0.35 to about 3.5 grams of (N0radical per liter, said aqueous solution being substantially completelyfree of sulfate ions.

2. A composition as claimed in claim 1 wherein the inorganicnitrogen-containing compound is selected from the group consisting ofnitric and nitrous acid and the water soluble alkali metal, alkali earthmetal and ammonium salts thereof.

3. A composition as claimed in claim 1 wherein the fluoride-containingcatalyst is selected from the group consisting of hydrofluoric acid,fluoboric acid, fluosilicic acid, water soluble alkali metal, alkaliearth metal and ammonium salts of hydrofluoric, fluoboric andfluosilicic acid and the reaction product of a hexavalent chromiumcompound, an organic reducing agent and a fluoride-silicon compound.

4. A process for the electrodeposition of black, chromium-containingdeposits which comprises making the electrically conductive member to beplated the cathode in an aqueous solution consisting essentially of fromabout 60 grams per liter to saturation of chromic anhydride, afluoride-containing catalyst in an amount sufiicient to supply fromabout 0.03 to about 1 gram of fluoride in solution per liter and aninorganic nitrogen-containing compound in an amount sufficient to supplythe equivalent of from about 0.35 to about 3.5 grams of (N0 radical perliter, said aqueous solution being substantially completely free ofsulfate ions, and passing direct current between said cathode and ananode immersed in said solution at a current density of from about 30 toabout 1500 amperes per square foot while maintaining said aqueoussolution at a temperature of from about 60 F. to about F.

5. A process as claimed in claim 4 wherein said inorganicnitrogen-containing compound is selected from the group consisting ofnitric and nitrous acid and the water soluble alkali metal, alkali earthmetal and ammonium salts thereof.

6. A process as claimed in claim 4 wherein said fluoride-containingcatalyst is selected from the group consisting of hydrofluoric acid,fluoboric acid], fluosilicic acid, water soluble alkali metal, alkaliearth metal and ammo nium salts of hydrofluoric, fluoboric andfluosilicic acid and the reaction product of a hexavalent chromiumcomcompound.

7. An article electroplated by the process of claim 4.

References Cited UNITED STATES PATENTS Ungelenk et al. 204-51 Schlijtter20451 Keuffel et a1.

Gilbert et a1 204-5l Smith 204-51 Smith 204---51 8 FOREIGN PATENTS624,887 8/1961 Canada. 881,596 7/1953 Germany.

22,105 10/1964 Japan. 5 135,319 5/1960 U.S.S.R.

OTHER REFERENCES Griffin, John L., Experimental ChromiumElectrodeposition as Affected by a Variety of Possible Catalysts, 10Plating, v01. 53, N0. 2, pp. 196203, February 1966.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

1. A COMPOUND OF MATTER FOR USE IN THE ELECTRO-DEPOSITION OF BLACK,CHROMIUM-CONTAINING DEPOSITS WHICH IS AN AQUEOUS SOLUTION CONSISTINGESSENTIALLY OF FROM ABOUT 60 GRAMS PER LITER TO SATURATION OF CHROMIXANHYDRIDE, A FLUORIDE-CONTAINING CATALYST IN AN AMOUNT SUFFICIENT TOSUPPLY FROM ABOUT 0.03 TO ABOUT 1 GRAM OF FLUORIDE IN SOLUTION PERLITER, AN INORGANIC NITROGEN-CONTAINING COMPOUND IN AN AMOUNT SUFFICIENTTO SUPPLY THE EQUIVALENT OF FROM ABOUT 0.35 TO ABOUT 3.5 GRAMS OF (NO2)RADICAL PER LITER, SAID AQUEOUS SOLUTION BEING SUBSTANTIALLY COMPLETELYFREE OF SULFATE IONS.